Cerium-based conversion coatings to improve the corrosion resistance of aluminium alloy 6061-T6

Corrosion Science xxx (2014) xxx–xxx

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Cerium-based conversion coatings to improve the corrosion resistance of aluminium alloy 6061-T6 B. Valdez a, S. Kiyota a, M. Stoytcheva a, R. Zlatev a, J.M. Bastidas b,⇑ a b

Engineering Institute, Autonomous University of Baja California (UABC), Blvd. Benito Juarez and Normal St., 21280 Mexicali, Baja California, Mexico National Centre for Metallurgical Research (CENIM), CSIC, Ave. Gregorio del Amo 8, 28040 Madrid, Spain

a r t i c l e

i n f o

Article history: Received 9 January 2014 Accepted 5 June 2014 Available online xxxx Keywords: A. Alloy A. Aluminium A. Rare earth elements B. Cyclic voltammetry B. EIS C. Paint coatings

a b s t r a c t Cerium-based conversion coatings were deposited on aluminium alloy 6061-T6 by immersion in two cerium salt sources (chloride- and nitrate-based) assisted with hydrogen peroxide (H2O2). The morphology and composition of the coatings were analysed using scanning electron microscopy and energy dispersive X-ray spectroscopy. Electrochemical measurements to assess corrosion behaviour were performed using free corrosion potential, polarisation and electrochemical impedance spectroscopy with a 3% NaCl solution. The influence of H2O2 on the generation of the coating was studied by cyclic voltammetry tests. The protective properties of the coating generated are heavily dependent upon the chelating effect, chaotropic anion, the pH and H2O2 content. Ó 2014 Elsevier Ltd. All rights reserved.

1. Introduction Aluminium alloys are widely used in different industries such as automotion, desalination, architecture, and in particular the aerospace sector due to their low cost and their excellent strength-to-weight ratio and corrosion resistance [1]. However, the heterogeneous microstructure of aluminium alloys makes them particularly liable to localised corrosion, limiting their applications especially in marine environments. Aluminium corrosion involves the adsorption of chloride ions on the metal surface and their reaction with aluminium in the oxide layer, diminishing its thickness [2]. Conversion coatings are used in metal finishing to afford corrosion protection and to improve the adhesion of paint systems to the underlying metal. Chromate conversion coatings (CrCC) have been used as anticorrosive treatments for aluminium, tin, zinc and steel. The corrosion protection of aluminium alloy components is generally provided by CrCC, primer and paint systems, where each film plays a specific role in the protection mechanism. Despite its undeniable qualities and advantages, CrCC is a simple chemical dip treatment process in a sodium dichromate solution which needs to be replaced due to the highly toxic and carcinogenic nature of hexavalent chromium [3–5]. Despite intense research on this subject, no equivalent replacement treatment has yet been found [6,7]. Environmentally-friendly alternatives to CrCC have been ⇑ Corresponding author. Tel.: +34 91 553 8900; fax: +34 91 534 7425. E-mail address: [email protected] (J.M. Bastidas).

investigated as potential replacements and include anodising [8], rare earth inhibitors and coatings [9], and sol–gel, among others [10–12]. It has been reported that rare-earth ions such as Ce, La, Pr, Nd [13], and Y [14] provide exceptional resistance to localised corrosion through the formation of insoluble hydroxide/oxide layers. Because of their low toxicity, rare-earth salts are not considered a health hazard. Consequently, coatings containing Ce and other rare-earth elements have been recommended as potential replacements for chromate-based formulations in metal finishing of aluminium alloys [15–17]. One of the most studied systems proposed by Hinton et al. [18] consists of salts with rare-earth ions that form insoluble hydroxides with exceptional resistance to localised corrosion [19,20]. In an aqueous solution of Ce(III) ions with hydrogen peroxide (H2O2), Ce deposition occurs rapidly at cathodic sites due to the local pH increase produced by the chemical reduction of H2O2 [21–25]. The aim of this paper is to assess the corrosion behaviour of Ce conversion coatings (CeCC) deposited on the aluminium alloy 6061-T6, assessing the effect of using chloride- or nitrate-anion Ce sources, the pH of the conversion solution, and the addition of hydrogen peroxide. The corrosion performance of the alloy 6061T6 treated with Ce conversion coatings has not been systematically explored in the literature. The coating morphology and composition were determined using scanning electron microscopy (SEM) and energy dispersive X-ray (EDX) spectroscopy. Corrosion potential (Ecorr), polarisation curves and electrochemical impedance

http://dx.doi.org/10.1016/j.corsci.2014.06.023 0010-938X/Ó 2014 Elsevier Ltd. All rights reserved.

Please cite this article in press as: B. Valdez et al., Cerium-based conversion coatings to improve the corrosion resistance of aluminium alloy 6061-T6, Corros. Sci. (2014), http://dx.doi.org/10.1016/j.corsci.2014.06.023

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spectroscopy (EIS) methods were used to study the corrosion behaviour assisted with adhesion and porosity results. 2. Experimental Specimens of tempered aluminium alloy 6061-T6 of dimensions 2.5 cm  3.7 cm, with the nominal composition (% by weight): 0.8– 1.2 Mg, 0.4–0.8 Si, 0.15–0.4 Cu, 0.04–0.35 Cr, 0.70 Fe, 0.15 Mn, 0.25 Zn and 0.2 of impurities were tested. Before conversion coating treatment and to favour the formation of Ce coatings, the specimens were mechanically ground down to 1200 grit with silicon carbide (SiC) paper. Chemical pretreatment was performed at room temperature and consisted of: (i) rinsing with ethanol, (ii) ultrasonic rinsing in acetone for 10 min, (iii) cleaning by immersion in an alkaline solution consisting of 50 g L1 of sodium hydroxide for 10 s, and (iv) etching in a 0.8 mol L1 of nitric acid solution for 10 min, to prevent surface defects and achieve uniform surface hydroxides/oxides. After alkaline cleaning and activation, the specimens were rinsed with deionised water. Table 1 includes the two aqueous solutions used as Ce sources for the chemical conversion coating treatments. Both solutions (A and B) were prepared at 25±1 °C, gently stirred for 5 and 10 min and used immediately after the H2O2 addition. The pH of the solutions was measured using a Thermo Orion potentiometer. The treated specimens were finally rinsed and dried at room temperature for 24 h before the electrochemical tests. It should be noted that the chemical pretreatment, coating temperature, reagent concentrations and immersion time were previously optimised in the laboratory. Five types of specimens were studied: specimen 6061-T6 was the reference aluminium alloy (base material) without conversion treatment; specimens M1 and M2 were treated with Solution A for 5 and 10 min, respectively; and specimens M3 and M4 were treated with Solution B for 5 and 10 min, respectively. The chemical composition and morphology of the base material (6061-T6) and the treated specimens (M1, M2, M3 and M4) were established using a SEM equipment a JEOL JSM-6360, JEOL, Peabody, MA, USA, coupled with an EDX analyser, an AMETEK, Mahwah, NJ. Prior to the electrochemical measurements, the Ecorr was monitored for 10 min of exposure to a 3% NaCl supporting electrolyte solution at pH 7, sufficient time to achieve a steady Ecorr. Three electrochemical techniques were used: (i) Ecorr was measured at different times; (ii) potentiodynamic polarisation curves were recorded from 0.5 V vs. Ag/AgCl below the Ecorr to +0.7 V vs. Ag/ AgCl above the Ecorr at a polarisation scan rate of 1 mV s1; and (iii) EIS measurements were made in a frequency range from 64 kHz to 10 mHz with a logarithmic sweeping frequency of 5 points per decade. For EIS, the test material was subjected to a 10 mV r.m.s. amplitude sine wave and EIS data was generated at the Ecorr. The testing time for EIS measurements was 1, 72 and 168 h. The corrosion current density (icorr) was obtained from the

Table 1 Chemical composition of the conversion treatment solutions used. Conversion treatment

Concentration (mol L1)

Compound

Chemical formula

Solution A

0.02

Cerium chloride heptahydrate Hydrogen peroxide

CeCl37H2O

Solution B

0.015

Cerium nitrate hexahydrate Hydrogen peroxide

Ce(NO3)36H2O

0.029 (30 vol.%)

0.029 (30 vol.%)

H2O2

H2O2

Fig. 1. Schematic representation of the cell used for electrochemical measurements. O-ring glass tube containing the 3% NaCl solution, counter and reference electrodes.

polarisation curves using the Tafel extrapolation method and from the EIS data. Additionally, cyclic voltammetry (CV) was performed to study the role of H2O2, from 0.5 V vs. Ag/AgCl to 2.0 V vs. Ag/ AgCl using at a scan rate of 10 mV s1. The electrochemical measurements were carried out with a PC4 FAS1 potentiostat from Gamry Instrument Inc., controlled by CMS100 and CMS300 software packages. A conventional three-electrode cell was used with a working electrode surface area of 1 cm2, a large area (4  9 cm2) platinum gauze as counter electrode, and an Ag/AgCl 3 M KCl as the reference. An O-ring glass tube containing the 3% NaCl solution and the counter and reference electrodes was fixed on the specimens, Fig. 1. Adhesion tests were performed according to ASTM D3359-08 standard [26]. A ScotchÒ PG 24 adhesion tape of 5.1 cm width was applied for 90 s. The tape was pulled off at an angle of close to 180° from the metal surface. 3. Results and discussion Visual inspections were performed after the chemical conversion coating treatment. Specimens M1 and M2 exhibited a gold– yellow to yellow colour, which indicates a hydrated Ce(III)/Ce(IV) oxide [27], whereas specimens M3 and M4 showed a pale yellow hue. The change in colour was interpreted as a result of the concentration of Ce hydroxides/oxides conforming the coating. After 24 h the specimens were reinspected and there was a reduction in the colour intensity, which may be attributed to a reduction in the hydration of the coating [28]. Fig. 2 shows SEM micrographs for the base material (6061-T6) and specimen M1 by way of example. Fig. 2a shows surface irregularities originated in the mechanical grinding process of the base material. The EDX analysis confirms the presence of Ce, O and Al in the conversion coating, see Table 2, and are in good agreement with the results obtained by other authors [29]. It is noteworthy that coatings generated with Solution A (specimens M1 and M2) generally possess a higher concentration of Ce and O than those obtained with Solution B (specimens M3 and M4), Table 2. Fig. 2b shows the Ce-rich particles and clusters. In general, the coatings consisted mainly of different sized spherical particles, in which larger particles were dotted with smaller ones, i.e. the coating was initially formed by the deposition of small particles, occasionally cracked, which grow and integrate to form larger

Please cite this article in press as: B. Valdez et al., Cerium-based conversion coatings to improve the corrosion resistance of aluminium alloy 6061-T6, Corros. Sci. (2014), http://dx.doi.org/10.1016/j.corsci.2014.06.023

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the corrosion protection performance [34–36]. The microstructure of this alloy differs from other alloys and it impacts the coating properties [12,37–42]. According to the model proposed by Hinton and Wilson for H2O2-assisted solutions, often acidified Ce salt solutions with added H2O2 [43], the acceleration effect of H2O2 probably is due to its reduction at cathodic sites generating hydroxyl ions (OH) [24,44]:

(a)

H2 O2 ðaqÞ þ 2e ¡2OH ðaqÞ

ð1Þ

the OH ions generated in reaction (1) increase the pH of the solution above the solubility limit of Ce species in solution, and favour the precipitation of Ce hydroxides/oxides on the aluminium surface:

Ce3þ ðaqÞ þ 3OH ðaqÞ ! CeðOHÞ3 ðsÞ

(b)

ð2Þ

In this model, H2O2 also plays an additional role as oxidant, transforming Ce(III) to Ce(IV) in solution by the formation of a complex:

Ce rich zones

2Ce3þ ðaqÞ þ H2 O2 ðaqÞ þ 2OH ðaqÞ¡2CeðOHÞ2þ 2 ðaqÞ

ð3Þ

leading to the formation of a hydroxide film containing mainly Ce(IV) [45]:

Fig. 2. SEM micrographs after the conversion coating treatment for (a) untreated specimen 6061-T6, and (b) specimen M1.

Table 2 EDX analysis of cerium conversion coatings deposited on aluminium alloy 6061-T6. Specimen

Element

Weight (%)

Atomic (%)

Untreated 6061-T6

O, K Al, K

7.63 92.37

12.23 87.77

M1

O, K Ce Al, K

6.88 6.62 86.50

11.68 1.28 87.03

M2

O, K Ce Al, K

3.96 4.77 91.27

6.76 0.93 92.31

M3

O, K Ce Al, K

4.69 3.38 91.93

7.87 0.65 91.48

M4

O, K Ce Al, K

3.59 4.42 91.99

6.13 0.86 93.01

particles [30–32]. As can be seen in Fig. 2b, specimen M1 possesses a dense and porous protective coating made up of uniform and homogeneous Ce agglomerates covering almost the whole surface. Furthermore, the white and grey trend of the Ce nodules visualised is related to the presence of a mixture of Ce hydroxides/oxides [33]. The cerium deposits star to grow over the cathodic sites formed by the intermetallic compounds of the aluminium matrix, due to a local pH increasing caused by the hydrogen peroxide reduction, in addition to the higher pH values of the conversion solution (pH 5.5 to 4) that leads to the formation of additional hydroxyl ions. The growth of cerium deposits blocks the cathodic zones effectively reducing the cathodic current of the system and improving

 CeðOHÞ2þ 2 ðaqÞ þ 2OH ðaqÞ ! CeO2  H2 OðsÞ þ H2 O

ð4Þ

CeðOHÞ2þ 2 ðaqÞ

ð5Þ



þ 2OH ðaqÞ ! CeO2 þ 2H2 O

Apart from accelerating the coating kinetics, H2O2 also increases the adhesion of the coating to the substrate and its corrosion resistance performance. It has been found that, in the Ce conversion coating reaction, H2O2 acts as a complexing agent, an oxidant, a crystallisation inhibitor and a source of OH, resulting in the precipitation reaction [46]. The growth of Ce deposits blocks the cathodic zones, effectively reducing the cathode area and therefore reducing the cathodic current of the system and improving its corrosion protection performance [34,36]. The pH of the solution is one of the most critical coating parameters. Most CeCC studies employ solutions adjusted to pH 2, since Ce species are more soluble at low pH values [47,48]. Fig. 3 shows the pH variation vs. time for Solutions A and B after H2O2 addition. It can be observed that the Ce nitrate solution initially possesses a lower pH than the Ce chloride solution. Moreover, once H2O2 is added, the pH of both solutions gradually decreases. The

Fig. 3. Evolution of pH with time for Solution A (CeCl37H2O assisted with H2O2) and Solution B (Ce(NO3)36H2O assisted with H2O2).

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Additionally, it should be considered the potential reaction of hydrogen peroxide itself. In an acid electrolyte can be reduced to water:

H2 O2 ðaqÞ þ 2Hþ þ 2e ! 2H2 O

ð10Þ

or in alkaline electrolyte oxidised to oxygen, which proceeds in two steps, the first being:

H2 O2 ðaqÞ þ OH ðaqÞ ! HO2 ðaqÞ þ H2 O

ð11Þ

which is the acid–base reaction between the weak acid (H2O2) and the alkaline electrolyte. The second is:

HO2 ðaqÞ þ OH ðaqÞ ! O2 þ H2 O þ 2e

Fig. 4. Cyclic voltammograms for (a) CeCl37H2O solution without H2O2, and (b) CeCl37H2O solution assisted with H2O2 (Solution A).

fluctuation in the pH, probably attributable to the instability of H2O2, is similar in both solutions, and after 10 min neither of the two solutions reach a constant value [49]. The pH of Solution B remained slightly higher than that of Solution A. The pH ranged from 4.45 to 4.04 for 5 min and from 4.45 to 3.91 for 10 min experimentation time. H2O2 starts to decompose rapidly if there are any active metals present, such as Fe/Cu that may dissolve into solution during coating process [50,51]. Ce in contrast, does not decompose peroxide [52]. Fig. 4 shows cyclic voltammograms for CeCl37H2O solution with H2O2 (Solution A) and without H2O2. The change in current density was not significant for CeCl37H2O solution without H2O2 (Fig. 4a), since oxygen and water reduction reactions:

O2 ðaqÞ þ 2H2 O þ 4e ¡4OH ðaqÞ

ð6Þ

2H2 O þ 2e ¡2OH þ H2

ð7Þ

do not provide the necessary amount of OH ions to lead an evident hysteresis in the reverse scan [53–63]. On the other hand, a remarkable current density variation for the H2O2-assisted solution (Solution A) can be observed (Fig. 4b). The cathodic peak around 1.2 V vs. Ag/AgCl in the reverse scan is related to reaction (1), revealing the role of H2O2 in producing larger amounts of OH ions. In this way, H2O2 tends to catalyse the production of OH ions and promotes a faster precipitation of Ce(OH)3(s), reaction (2) [35]. In addition, the oxidising effect of H2O2 allows part of the Ce(III) to be converted to Ce(IV), generating the hydroperoxyl (–OOH) group [44,64]:

2CeðOHÞ3 ðsÞ þ 3H2 O2 ¡2CeðOHÞ3 ðOOHÞ þ 2H2 O

ð8Þ

CeðOHÞ3 ðOOHÞ þ 2CeðOHÞ3 ðsÞ þ H2 O ! 3CeðOHÞ4 ðsÞ

ð9Þ

Even so, thermodynamically the evolution of CeO2 is more favoured from Ce(OH)3 than from Ce(OH)2+ [49]. Hydrogen 2 peroxide causes the solution to change from colourless to a yellow–orange colouring, which is related to the formation of hydroxyl–cerium complexes [65,66].

ð12Þ

These electrochemical reactions can take place directly on the surface of the system. The ionisation constant of Ce chloride is low in contrast to the polyatomic anion of Ce nitrate, Table 3, enhancing the affinity of chelating ligands to Ce(III) ions [67]. Nitrate ions are bidentate ligands with high coordination numbers, and so more stable com3 plexes with Ce(III) are formed such as Ce(NO3)2 5 or Ce(NO3)6 . One part of the salt is hydrolysed in Ce(III), and the other part forms complexes due to the presence of oxygen in the polyatomic anion:

2CeðNO3 Þ3 ¡Ce3þ þ CeðNO3 Þ3 6

ð13Þ

Therefore, the presence of these stable chelates reduces the concentration of Ce available to generate insoluble products, leading a reduction in the corrosion resistance of the protective coating. Nitrate ions also help to passivate the surface of the aluminium alloy via the incorporation of alumina (Al2O3) in the surface oxide film [44]. As a result, a lower concentration of Ce(III) is available in the solution to form insoluble products that will become the Ce-based conversion coating. In the case of the CeCl37H2O compound, chloride ions are monodentate ligands that form less stable and weekly bonding complexes. Thus, this compound can be easily ionised: 

2CeCl3 ¡2Ce3þ þ 6Cl

ð14Þ

and subsequently the amount of Ce(III) ions available to react with the OH ions generated during the cathodic reaction is higher than with the Ce(NO3)6H2O compound, favouring the formation of a conversion coating with better anticorrosive properties [68–70]. Table 4 includes adhesion test results performed according to ASTM D3359-08 standard [26]. The adhesion results indicate that for a given treatment time Solution A (specimens M1 and M2) generates a more adherent coating than Solution B (specimens M3 and M4). Additionally, as the treatment time increases for a given solution from 5 to 10 min the adhesion behaviour is improved. Fig. 5 shows the Ecorr variation vs. time for the untreated and treated specimens in contact with the 3% NaCl solution. It can be observed that the four treated specimens have more stable and nobler potentials over time than the untreated specimen (6061T6) in a range of 50–200 mV vs. Ag/AgCl, which may be attributed to the protection level afforded by the coating, probably because the Ce compounds suppress the cathodic oxygen reduction, reaction (6) [13,17].

Table 3 Ionisation constant for Ce nitrate and Ce chloride [67]. Anion

Coordination number

Geometric form of the ligand atom positions

Geometric form of the media position of ligand

Metal complex

Complex with Ce(III) formation constant (pK1, pK1,2)

NO 3

10 12

Trigonal antiprism Icosahedron

Trigonal bipyramid Octahedron

Ce(NO3)2 5 Ce(NO3)3 6

1.04 1.51

Cl

–

–

–

–

0.22

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B. Valdez et al. / Corrosion Science xxx (2014) xxx–xxx Table 4 Adhesion test results, according to ASTM D3359-08 standard [26]. Conversion treatment

Specimen

Treatment time (min)

Classification

Percent area removed (%)

Adhesion

Solution Solution Solution Solution

M1 M2 M3 M4

5 10 5 10

4B 5B 2B 4B

<5 0 15–35 <5

Good Good Moderate Good

A A B B

Fig. 5. Corrosion potential (Ecorr) measurements vs. time for the bare metal (6061T6) and for treated specimens in contact with 3% NaCl solution for 1 h.

Fig. 6 shows polarisation curves for the untreated and treated specimens. Polarisation measurements were carried out after 1 h in contact with the 3% NaCl solution. The treated specimens showed more positive Ecorr values than the untreated specimen, indicating that the Ce-based conversion coatings ennoble the potential and act as anodic inhibitors, improving corrosion resistance, which is consistent with the Ecorr results (Fig. 5). Nevertheless, the ennoblement of potential for specimens M3 and M4 was not significant as in the case of specimens M1 and M2 treated with Solution A, particularly, where it is favoured for longer treatment times (10 min: specimen M2). Table 5 summarises the Ecorr, polarisation and EIS results for 1 h experimentation. The icorr was obtained from polarisation measurements and from EIS data using RP parameter. A considerable improvement was obtained in the

Fig. 6. Polarisation curves for the bare metal (6061-T6) and the treated specimens in contact with 3% NaCl solution for 1 h.

icorr value comparing the base material (5.1 or 4.8 lA cm2) and the treated specimens, which decreased by around one order of magnitude. Specimens M1 and M2 presented the lowest icorr value, 0.4 or 0.8 lA cm2 and 0.1 lA cm2, respectively. In contrast, specimens M3 and M4 had the highest icorr value, 4.0 or 1.7 lA cm2 and 0.8 or 0.3 lA cm2, respectively, which explains why the specimens treated with Solution B generate less protective coatings than Solution A, being less compact, more porous and less adherent [71]. Figs. 7–9 show typical Bode plots for uncoated substrate and specimens M1, M2, M3 and M4 (Fig. 7) with comparative results for 1 h experimentation, and specimens M2 (Fig. 8) and M4 (Fig. 9), as two examples, in contact with the 3% NaCl solution for 1, 72 and 168 h. Two time constants are defined (see Fig. 7). The relaxation process at high-intermediate frequencies, associated with the coating properties, presented a phase angle (h) close to 90°, which indicates a capacitive behaviour with good dielectric properties, i.e. the conversion coatings have the ability to charge, avoiding the ionic flow of the corroding solution [33,62]. In contrast, the untreated specimen 6061-T (see Fig. 7) showed a h value slightly higher than 70°, since its capacitive properties are poorer than those of the treated specimens. Additionally, as the frequency decreased, the phase angle tended to drop and led to a second relaxation process, related to the penetration of the 3% NaCl solution through the pore network of the coating reaching the underlying material surface [72]. The interpretation of EIS data depends largely on the electrical equivalent circuit (EEC) used to model the 6061-T6/CeCC/NaCl system. The fitting procedure was performed using the EEC of Fig. 10 and is formed by a first resistance (RS) modelling the ohmic electrolyte resistance, in series with a first constant phase element (CPE1) in parallel with a second resistance (RC) representing the coating properties, and in series with a second parallel subcircuit, that correlates the interaction between the base material and the 3% NaCl solution, through the pore network of the coating, representing the double-layer capacitance (CPE2) and the charge transfer resistance (RP) associated with the corrosion process [20], and for modelling the untreated aluminium alloy assuming the presence of a thin oxide film of alumina on the surface [73–75]. The physical meaning of a CPE is associated with a non-ideal capacitive behaviour involving the 6061-T6/CeCC/NaCl system. Thus, CPE parameters were used instead of capacitances in order to consider coating defects such as porosity, roughness, heterogeneous appearance, coating thickness, among others, which allow the corroding solution to cross the 6061-T6/CeCC interface [71,76,77]. The electrical impedance of a CPE is written as, ZCPE = [YP(jx)a]1, where x is the angular frequency (rad s1) equal to 2pf, f is the applied frequency (Hz), 2p is the habitual conversion constant, YP (S cm2 sa) and a (dimensionless) are the parameters of the CPE, the exponent a is 1 < a < +1. When a = 1, the CPE is a pure capacitor with a capacitance of YP, if a = 1, the CPE is an inductor, and when a = 0.5, the CPE is a Warburg impedance [76]. A complex non-linear least squares analysis was performed to fit the EEC parameters to the impedance data [78]. It should be indicated that there was good agreement between the experimental and the fitted results, the chi-squared (v2) error value was of the order of 103 for all cases, indicating that the EEC

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Table 5 Electrochemical parameters: corrosion potential (Ecorr), anodic (ba) and cathodic (bc) slopes determined by the covering natural interval, and corrosion current density (icorr) obtained from the Tafel extrapolation method and from EIS measurements. Specimen

Ecorr, V vs. Ag/AgCl

6061-T6 M1 M2 M3 M4

Polarisation measurements

0.811 0.546 0.602 0.767 0.758

bc, V vs. Ag/AgCl

icorr (lA cm2)

RP (kX cm2)

icorr (lA cm2)

Porosity

0.360 1.358 0.426 0.744 0.259

0.438 0.994 0.327 1.095 0.330

5.1 0.4 0.1 4.0 0.8

18.0 320.3 630.0 114.2 250.0

4.8 0.8 0.1 1.7 0.3

– 0.010 0.008 0.120 0.050

10 6

AA6061-T6 M1 M2 M3 M4

10 5

|Z|, Ω cm2

EIS measurements

ba, V vs. Ag/AgCl

10 4 10 3 10 2

Phase Angle, degrees

10 1 -90 -60 -30 0 10

-2

10

-1

10

0

10

1

10

2

10

3

10

4

10

5

Frequency, Hz Fig. 7. Bode plots for the uncoated substrate (6061-T6) and the specimens M1, M2, M3 and M4 in contact with 3% NaCl solution for 1 h experimentation.

Fig. 9. Bode plots for specimen M4 contact with 3% NaCl solution for 1, 72 and 168 h experimentation.

CPE1 RS CPE2 RC RP Fig. 10. Electrical equivalent circuit (EEC) used to fit impedance data.

Fig. 8. Bode plots for specimen M2 in contact with 3% NaCl solution for 1, 72 and 168 h experimentation.

used to model the 6061-T6/CeCC/NaCl system was adequate. The a1 and a2 exponents ranged from 0.69 to 0.98. The RS parameter showed values between 16 and 26 X cm2, indicating high conductivity of the 3% NaCl solution. After 1 h in contact with the 3% NaCl solution, specimens M1, M2, M3 and M4 showed RC values of 261.0, 324.0, 216.2 and 183.4 kX cm2, respectively, indicating a notable improvement in the protective properties of the coatings compared with the base material (2.0 kX cm2), see Fig. 11. The RC values decreased as

exposure time increased. For specimens M1 and M2 this variation was not significant because they retain the same order of magnitude after 168 h of exposure (147.2 and 169.2 kX cm2, respectively). Conversely, specimens M3 and M4 showed a more evident decrease in RC (25.1 and 79.1 kX cm2, respectively) due to the degradation of the coating and the impoverishment of its anticorrosive properties. The CPE1 values were approximately one order of magnitude lower (4.4–9.2 lS cm2 sa1), Fig. 11, in relation to the base material (80.3 lS cm2 sa1). The treated specimens in contact with the 3% NaCl for 1 h showed the highest charge transfer resistance (RP) by one order of magnitude compared to the untreated specimen 6061-T6 (18.0 kX cm2), see Fig. 7. Fig. 12 shows the variation in RP and Y2 parameters with time for specimens M1, M2, M3 and M4 in contact with the 3% NaCl solution. At first (1 h) the coatings showed good resistive and capacitive behaviour, since their properties are not yet affected by the aggressiveness of the 3% NaCl solution. The RP parameter for the specimens treated with Solution A was high for the longest treatment time, specimen M2 (10 min) had a RP of

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B. Valdez et al. / Corrosion Science xxx (2014) xxx–xxx

M1 M2 M3 M4

(a)

11

Y1 / μS cm-2 s α1

10 9 8 7 6 5

(b) R c / k Ω cm2

300

200

100

0

20

40

60

80

100 120 140 160 180

Time / h Fig. 11. Variation in RC and Y1 parameters with time for specimens M1, M2, M3 and M4 in contact with 3% NaCl solution.

1000

(a)

Y2 / μS cm-2 s α2

800 600 400 200

(b)

M1 M2 M3 M4

R p / k Ω cm2

600

400

200

0

0

20

40

60

80

100 120 140 160 180

Time / h

7

decreased, allowing the corroding solution to reach to the underlying material surface. The variation in R3 may be associated with the porous and/or non-homogenous nature of the coating, as well as the changes in the thickness, homogeneity and density of the coating taking place within the microstructure [21,43]. In this way, RP represents the corrosion resistance of the whole material. Furthermore, the value of Y2 parameter was lower for specimens M1, M2 and M4 than for specimen M3, see Fig. 12. This indicates that the practical corroded surface area is smaller for specimens M1, M2 and M4 than for M3 [47]. Moreover, Y2 had a tendency to increase, which implies a decrease in the dielectric properties of the coating. It should be noted that for 168 h of experimentation the capacitance value related with coating properties of specimen M3 is high, which may be associated with a poor protection. After impedance measurements were performed, the specimens were observed by optical microscopy (OM) and showed that specimens M2 and M1 did not seem affected by pitting corrosion, while specimens M3 and M4 showed small pits on the surface. Assuming that the coating is electrochemically inert at low overpotentials, the porosity of the coating can be estimated from electrochemical values using the empirical relation [79,80]:
DEcorr F ¼ Rp;m =Rp  10 ba , where F is the total coating porosity, Rp,m the polarisation resistance of the base material, Rp the measured polarisation resistance or the semicircle diameter at low frequencies using EIS data, DEcorr the difference between the corrosion potentials (coating and substrate) and ba the anodic Tafel slope of the base material. Very porous films (>0.01) show changes in the Ecorr [79]. Measuring the porosity appears to be an easy way to determine the film density [80]. Table 5 includes porosity results (last column) and electrochemical results for the four coatings tested. A porosity of 0.010 for M1, 0.008 for M2, 0.120 for M3 and 0.050 for M4 was yielded. The porosity is higher for coating M3 than for the other coatings (M3 > M4 > M1 > M2). The coating porosity is closely related to coating quality and directly influences its corrosion resistance. The appearance of porosity in closed and open voids and in pinholes causes the film density to be lower than that of the bulk material. The beneficial effect of low porosity is that it impedes the passage of corroding solution to the underlying material and reduces localised corrosion kinetics. Finally, water uptake was estimated using the empirical relat =C t¼0 Þ tion proposed by Brasher and Kinsbury [81]: V H2 O ¼ logðC , log 80 where V H2 O is the volume fraction of water in the coating, Ct is the coating capacitance at time t, and Ct=0 is the capacitance of the coating prior to water uptake at time t = 0. In this model, the coating capacitance is assumed that to only be affected by water uptake and 80 is the relative permittivity of water at T = 20 °C [82]. However, coating capacitance is also influenced by the chemical reaction between the coating and the corroding solution. Based on this relation, the volume fraction of water in the coating reached 0.12 for M4 and 0.06 for M3, which indicates the presence of water/electrolyte in the coating matrix and unsaturation of the pores or voids formed in the coating by the electrolyte. In contrast, specimens M1 and M2 yielded erratic (negligible) results. These results are in agreement with electrochemical, adhesion and porosity results.

Fig. 12. Variation in RP and Y2 parameters with time for specimens M1, M2, M3 and M4 in contact with 3% NaCl solution.

4. Conclusions

630 kX cm2, while specimen M1 (5 min) had a RP of 320.3 kX cm2, whereas in the case of the specimens treated with Solution B, specimen M3 (5 min) had a RP of 114.2 kX cm2 while specimen M4 (10 min) had a RP of 250 kX cm2, see Fig. 12. In general, RP tended to decrease as the testing time in contact with the 3% NaCl solution increased, and hence the anticorrosive properties

Cerium conversion coatings on aluminium alloy 6061-T6 were achieved using Ce chloride and nitrate aqueous solutions assisted with hydrogen peroxide (H2O2): Solution A (CeCl37H2O with H2O2) and Solution B (Ce(NO3)36H2O with H2O2). Four types of specimens were prepared: specimens M1 and M2 by immersion in the Solution A for 5 and 10 min, respectively; and specimens M3 and M4 by immersion in Solution B for 5 and 10 min,

Please cite this article in press as: B. Valdez et al., Cerium-based conversion coatings to improve the corrosion resistance of aluminium alloy 6061-T6, Corros. Sci. (2014), http://dx.doi.org/10.1016/j.corsci.2014.06.023

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B. Valdez et al. / Corrosion Science xxx (2014) xxx–xxx

respectively. Specimen M2 presented the highest Ce concentration, and exhibited the greatest coverage by a denser coating. Porosity results were from high to low M3 > M4 > M1 > M2. The increase in corrosion resistance was higher for the coating generated with Solution A than with Solution B. Specimen M2 shows the best corrosion resistance, with a charge transfer resistance (Rp) of 630 kX cm2 and a corrosion current density (icorr) of 0.1 lA cm2. In contrast, specimen M3 showed the poorest corrosion resistance, Rp of 114.2 kX cm2 and icorr  2 lA cm2. The untreated base material (6061-T6) showed a Rp of 18 kX cm2 and icorr  5 lA cm2. The Rp values for specimens M3 and M4 were in the range 22.4–250 kX cm2 after 1, 72 and 168 h of exposure to the 3% NaCl solution. The coating corrosion protection was associated with the chelating properties of the anion, i.e. the number of complexes that the anion is capable of forming. Cyclic voltammetry tests showed that H2O2 has an important role in the production of hydroxyl ions catalysing the precipitation of Ce hydroxides/oxides. Acknowledgment The authors wish to thank the Consejo Nacional de Ciencia y Tecnología (CONACYT) of Mexico for the financial support of this work. References [1] S.M. Cohen, Replacements for chromium pretreatments on aluminium, Corrosion 51 (1995) 71–78. [2] H. Hasannejad, T. Shahrabi, A.S. Rouhaghdam, M. Aliofkhazraei, E. Saebnoori, Investigation of heat-treatment and pre-treatment on microstructure and electrochemical properties of cerium nano-oxide films on AA 7020 by sol–gel methods, Appl. Surf. Sci. 254 (2008) 5683–5690. [3] K.A. Korinek, Chromate Conversion Coatings, ASM Handbook, Corrosion, vol. 13, American Society for Materials, Materials Park, OH, 1987. p. 389. [4] R.G. Buchheit, A.E. Hughes, Chromate and Chromate-Free Conversion Coatings, ASM Handbook, Corrosion: Fundamentals, Testing and Protection, vol. 13, American Society for Materials, Materials Park, OH, 2003. pp. 720–736. [5] S. Geng, S. Joshi, W. Pinc, W.G. Fahrenholtz, M.J. O’Keefe, T.J. O’Keefe, P. Yu, Influence of processing parameters on cerium based conversion coatings, in: Proc. Tri-Service Corrosion Conference, Denver, CO, NACE International, 2007, p. 10. [6] Y. Kobayashi, Y. Fujiwara, Effect of SO2 4 on the corrosion behaviour of ceriumbased conversion coatings on galvanized steel, Electrochim. Acta 51 (2006) 4236–4242. [7] T.M. Dobrev, M.C. Monev, I.N. Krastev, R.P. Zlatev, Electrochemical methods for evaluation of the protective ability of electroplated coatings and conversion films, Bulg. Chem. Commun. 40 (2008) 198–203. [8] B. Meyers, S. Lynn, ASM Handbook, vol. 5, American Society for Materials, Materials Park, OH, 1994. p. 925. [9] Y. Xingwen, C. Chunan, Y. Zhiming, Z. Derui, Y. Zhongda, Study of double layer rare earth metal conversion coating on aluminium alloy LY12, Corros. Sci. 43 (2001) 1283–1294. [10] G.R. Salazar-Banda, S.R. Moraes, A.J. Motheo, S.A.S. Machado, Anticorrosive cerium-based coatings prepared by the sol–gel method, J. Sol–Gel. Sci. Technol. 52 (2009). 425–423. [11] B.Y. Johnson, J. Edington, M.J. O’Keefe, Effect of coating parameters on the microstructure of cerium oxide conversion coating, Mater. Sci. Eng. A 361 (2003) 225–231. [12] H.D. Johansen, C.M.A. Brett, A.J. Motheo, Corrosion protection of aluminium alloy by cerium conversion and conducting polymer duplex coatings, Corros. Sci. 62 (2012) 342–350. [13] D.R. Arnott, B.R.W. Hinton, N.E. Ryan, Cationic-film-forming inhibitors for the protection of the AA-7075 aluminum-alloy against corrosion in aqueous chloride solution, Corrosion 45 (1989) 12–18. [14] F. Mansfeld, Y. Wang, H. Shih, Evaluation of the surface properties of aluminium alloys by electrochemical methods, in: Corrosion 91, NACE, Cincinnati, OH, Paper no. 134, 1991. [15] B.R.W. Hinton, D.R. Arnott, N.E. Ryan, Cerium conversion coatings for the corrosion protection of aluminium, Mater. Forum 9 (1986) 162–173. [16] J. Hu, X.H. Zhao, S.W. Tang, M.R. Sun, Corrosion protection of aluminium borate whisker reinforced AA6061 composite by cerium oxide-based conversion coating, Surf. Coat. Technol. 201 (2006) 3814–3818. [17] J. Hu, X.H. Zhao, S.W. Tang, W.C. Ren, Z.Y. Zhang, Corrosion resistance of cerium-based conversion coatings on alumina borate whisker reinforced AA 6061 composite, Appl. Surf. Sci. 253 (2007) 8879–8884. [18] B.R.W. Hinton, D.R. Arnott, N.E. Ryan, The inhibition of aluminium alloy corrosion by cerous cations, Mater. Forum 7 (1984) 211–217.

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